Definition:Sigma-Ring/Definition 1
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Definition
A $\sigma$-ring is a ring of sets which is closed under countable unions.
That is, a ring of sets $\Sigma$ is a $\sigma$-ring if and only if:
- $\ds A_1, A_2, \ldots \in \Sigma \implies \bigcup_{n \mathop = 1}^\infty A_n \in \Sigma$
Also see
Linguistic Note
The $\sigma$ in $\sigma$-ring is the Greek letter sigma which equates to the letter s.
$\sigma$ stands for for somme, which is French for union.